Do you have the original belt? The best way to get a replacement is to buy from a lawn mower shop and take the old belt with you so they can measure it. Some machines(Murray for example) use very wierd metric sizes. The only place to get the right belt is from Murray.
Auto belts only work temporary. I use special blue belts from my local True Value hardware and they have worked well.
Do you have a model # of the machine or owners' manual with the belt part number?
If there is space, you could try using a slightly larger or smaller idler pulley. I would need to see the mechanics to see if there is a way to adjust it.